1. Field of the Invention
The present invention relates to nonreciprocal circuit devices such as isolators and circulators used in microwave bands and the like, and also relates to communication apparatuses incorporating the nonreciprocal circuit devices.
2. Description of the Related Art
Hitherto, a balun, a hybrid circuit, or a power synthesizer has been interposed on the output side of a balanced output circuit, particularly, on the output side of a push-pull amplifier having a pair of amplifiers driven with a phase difference of 180 degrees. With the balun or the like, a balanced signal has been converted into a single ended (unbalanced) signal.
In general, a balun can be used for the HF band, the VHF band, the UHF band and lower. On the other hand, in the UHF band (about 300 MHz-3 GHz), a hybrid circuit or a power synthesizer can also be used. In the case of a balun, a broadband ferrite core is often used. In this case, the usable highest frequency band is the UHF band. Usually, since both the hybrid circuit and the power synthesizer are distributed constant circuits, there is no practical problem with the use of the hybrid or the power synthesizer above the UHF band.
In the transmission circuit section of a communication apparatus, particularly in the transmission circuit section for QPSK modulation including an amplitude modulating component and in a transmission circuit section requiring high reliability, a transmission signal converted into a single ended signal passes through an isolator and then is transmitted to an antenna via an antenna switching device (or an antenna duplexer). In the case the isolator is not inserted in the transmission circuit, reflection from the antenna, the antenna duplexer, or the like returns to a balanced output circuit (particularly, an amplifier). Consequently, this changes load impedance viewed from the balanced output circuit. When the load impedance changes, the waveform of the transmission signal is greatly deformed. In addition, the amplifier s operation becomes unstable and oscillation occurs.
However, as in the case of the related art, when a balun, a hybrid circuit, or a power synthesizer is combined with an isolator, the transmission circuit section becomes larger and more expensive. This doesn""t meet the recent demand for miniaturization and cost reduction of a mobile communication apparatus. Furthermore, since a transmission signal passes through both the balun and the isolator, insertion loss increases. In addition, since a large amount of power flows in the transmission circuit section, many kinds of device are, generally speaking, necessary to safely and correctly control the power. Thus, unnecessary radiation tends to occur, which often causes mutual interference between the components inside the communication apparatus. Additionally, since the operational frequency bandwidth of the transmission circuit section is narrowed by the operational frequency bandwidths of both of the balun and the isolator, the usable frequency band becomes narrower.
In addition to the above problems, in the communication apparatus, in order to prevent the emission of second and third harmonic waves from a power amplifier, a low pass filter or a band pass filter is often included in addition to the isolator, to suppress a harmonic wave signal to a level of approximately xe2x88x9260 dB with respect to the fundamental wave ratio. However, in such a circuit structure, the size, the cost, and the insertion loss increase. Thus, it is desirable for the isolator to suppress harmonic waves effectively, thus avoiding the need for a filter.
It is known that an isolator can effectively suppress harmonic waves at frequencies higher than an operational frequency (fundamental wave). Particularly, the signals of waves deviating far from the fundamental wave, such as the signals of a third harmonic wave, can be sufficiently attenuated, for example, by 30 to 40 dB or more. However, primarily, an isolator is not generally used as a filter. Thus, the signals of frequencies relatively close to the fundamental wave, such as a frequency signal of a second harmonic wave, are attenuated by only 15 to 25 dB. This is not sufficient attenuation when compared with the attenuation of the third harmonic wave signals.
In a usual unbalanced (single ended) output amplifier, second harmonic wave signals are stronger (approximately xe2x88x9230 dB below the fundamental wave) than third harmonic wave signals (approximately xe2x88x9240 dB below the fundamental wave). Thus, when using only an isolator, the second harmonic wave signals cannot be sufficiently attenuated (approximately xe2x88x9250 dB below the fundamental wave). As a result, a filter is often needed to attenuate the second harmonic wave signals to be at xe2x88x9260 dB or lower with respect to the fundamental wave.
However, characteristically, in the balanced-type (push-pull) amplifier, such a second harmonic wave does not occur so often (example: approximately from xe2x88x9240 to xe2x88x9250 dB below the fundamental wave). Thus, it is more problematic to suppress a third harmonic wave (approximately xe2x88x9240 dB below the fundamental wave). On the other hand, since the isolator is highly capable of suppressing the third harmonic wave, as mentioned above, by combining the balanced amplifier and the isolator, the third harmonic wave will be sufficiently suppressed to approximately xe2x88x9260 dB below the fundamental wave without adding a filter (the second harmonic wave doesn""t become so big a problem because it rarely occurs in the balanced-type amplifier as described above). However, in the case of the related art, it is necessary to interpose a balun between the isolator and the balanced-type amplifier. (All of the conventional isolators have only unbalanced ports. Therefore a balun is inherently necessary in order to connect a conventional isolator and a balanced-type amplifier which have different types of ports.)
To address these issues, the present invention provides a nonreciprocal circuit device capable of being connected directly to a balanced output circuit without interposing therebetween a balun, a hybrid circuit, or the like. The invention further provides a communication apparatus incorporating the nonreciprocal circuit device.
To this end, according to a first aspect of the present invention, there is provided a nonreciprocal circuit device including a plurality of ports, a permanent magnet, a ferrite member to which the permanent magnet applies a DC magnetic field, and a plurality of central electrodes arranged on the ferrite member. In the nonreciprocal circuit device, at least one of the plurality of ports connected to the central electrodes is a balanced port. More specifically, both ends of the central electrode corresponding to the balanced port may be feeding ends. Preferably, each of the central electrodes corresponding to has an electric length of substantially xc2xd wavelength.
The nonreciprocal circuit device having the above structure can be connected directly to the output side of the balanced output circuit without interposing a balun, a hybrid circuit, or the like.
In addition, in order to obtain impedance matching between the nonreciprocal circuit device and the balanced output circuit connected thereto, for example, a matching capacitor may be electrically connected in series to each end of the central electrode of the balanced port, a matching capacitor may be electrically connected between the two ends of the central electrode of the balanced port, or a matching capacitor may be electrically connected between each end of the central electrode of the balanced port and a ground. Alternatively, each end of the central electrode of the balanced port may be electrically connected to a balanced input terminal via a matching capacitor, a matching capacitor may be electrically connected between the balanced input terminals, or a matching capacitor may be electrically connected between each of the balanced input terminals and the ground.
In addition, the width of the central electrode of the balanced port may differ from the widths of the remaining central electrodes. With this arrangement, an optimum impedance match can be obtained between the nonreciprocal circuit device and the balanced output circuit. Especially when the impedance of the balanced output circuit is low, the width of the central electrode of the balanced port may be set broader than the widths of the remaining central electrodes. Consequently, conductive loss at the central conductors can be reduced and thereby low insertion loss can be obtained in the nonreciprocal circuit device.
According to a second aspect of the present invention, there is provided a communication apparatus including the nonreciprocal circuit device of the invention and a pair of amplifiers driven with a phase difference of approximately 180 degrees. In the communication apparatus, the balanced port of the nonreciprocal circuit device is connected to the output side of the pair of amplifiers.
Other features and advantages of the present invention will become apparent from the following description of embodiments of the invention which refers to the accompanying drawings.